New precise W boson mass measurement surprises and reassures physicists

Scientists have just measured the mass of the W boson. It's different from what they hoped - but also exactly what they always thought it should be.

Based on the theories of particle physics, there are four forces in the universe: gravity, electromagnetism, weak force and strong force. Bosons mediate these forces. The W boson is one of the cornerstones of the Standard Model, the theoretical framework that describes nature at its most fundamental level.

A precise understanding of the W boson mass allows scientists to map the interplay of particles and forces, including the strength of the Higgs field and merger of electromagnetism with the weak force, which is responsible for radioactive decay. Any changes to the mass of the W boson would mean that there must exist undiscovered physics close to the energies that scientists can already probe.

A prior attempt at measuring the mass found it to be extremely different from the theory, which lit a fire in the physics world to figure out what kind of new physics were going on. That may have all been a tempest in a teapot.

In an incredible feat of precision, physicists have now accurately measured the mass of the W boson and discovered that it is different from the previous precise measurement. They found that the W boson's mass is 80360.2 ± 9.9 megaelectron volts, or MeV, which is consistent with the Standard Model's predictions of 80 357 ± 6 MeV. Another way of saying this is that the W boson's mass is 80 giga electron volts or 1.42e-25 kilograms.

The mass of fundamental particles is measured in electron volts because from Einstein's E=mc^2, scientists know that mass and energy are related to the speed of light. An electron volt is the energy of an electron accelerated by a voltage of 1V.

"Everybody was hoping we would measure it away from the theory, igniting hopes for new physics," said Michalis Bachtis, a UCLA associate professor of physics whose research group played a key role in the experiment. "By confirming that the mass of the W boson is consistent with the theory, we have to search for new physics elsewhere, maybe by studying the Higgs boson with high precision as well."

The new results were obtained by the Compact Muon Solenoid, or CMS, experiment at CERN's Large Hadron Collider. The instruments for this experiment have a unique compact design, specialized sensors for muons and an extremely strong solenoid magnet that bends the trajectories of charged particles as they move through the detector. Important parts of the CMS detector have been built on the UCLA campus since the 1990s.

"Basically, we used a 14,000-ton scale to measure the weight of a particle that has a mass of 1x10^-25 kg, or about 80 times the mass of a proton," said Bachtis, who was also involved in the discovery of the Higgs boson.

Because most fundamental particles are incredibly short-lived, scientists measure their masses by adding up the masses of everything they decay into in a subatomic version of the desk toy Newton's Cradle. This method works well for particles like the Z boson, a cousin of the W boson, which decays into two muons, which are relatively easy to measure. But the W boson poses a big challenge because one of its decay products is a tiny fundamental particle called a neutrino, which is notoriously difficult to measure.

This specific measurement was considered impossible at the LHC because of the need to calibrate the energy of the muons, which are heavier siblings of electrons, with only a 0.01% margin of error, considered impossible before in such a complex device. Bachtis and postdoctoral researcher Elisabetta Manca have been working to achieve this exceedingly fine level of detail for the last eight years.

"I started this research as a summer student, and now I'm in my third year as a postdoc," Manca said. "It's a marathon, not a sprint."

"This new level of precision will allow us to tackle critical measurements, such as those involving the W, Z and Higgs boson, with enhanced accuracy," Manca said.

The research had to overcome some unexpected challenges.

"We found out that the magnetic field of the experiment changed significantly when the detector was lowered in the cavern 100 meters underground compared to the surface," Bachtis said. "This was negligible for most measurements but not for the W boson mass. These small variations matter. Our analysis also had to correct for the deformation of the detector by its own gravity."

With the new W boson measurement, scientists believe that undiscovered physics may hide in different places. Now it's time to roll up their sleeves and continue exploring.

"Now we need to both exploit the maximum potential of our current experiment, but also start designing the next big experiment to be able to directly or indirectly reach higher energies in order to make new physics discoveries," Bachtis said.